It's not possible to construct a regular 13 sided polygon (tridecagon or triskaidecagon) using a straightedge and a compass. However, you can get so close that you won't be able to see the difference. This video shows you how.
The polygon gets (almost) constructed inside a given circle, that becomes the circumcircle of the polygon.
For more videos and math help, see http://www.dr-mikes-math-games-for-kids.com/
Music "Go Cart" by Kevin MacLeod, http://www.incompetech.com